Suppresing render console output
Unfortunately, the message you want to get ride (Fra:5...) is printed directly on stdout by a fprintf function (source/blender/render/intern/source/pipeline.c). It's not printed by a specific function which has a hook with python... And I don't think the embedded Python engine has the power to act on the pipe between the console and Blender. I think the only way to do what you want is to write an external launcher script to filter Blender stdout. As you don't want to use external tools like awk, you can use the Blender python engine which is in the blender directory.